Effects of attenuator on the flux for the LAM data

Flux in the LAM data is quite high - higher than in the data taken last in December 2017. We have recently realized that we want to have exposure times of at least 10 sec (probably closer to 15) to minimize effects of the shutter transit on the PSFs. This requires the use of the attenuator. The ticket is to estimate how does the flux change as a function of the variable ``attenuator'' in https://people.lam.fr/madec.fabrice/pfs/ait_logbook_SM1.html.

Overview.png gives overview of the behavior of the flux, as a function of attenuator. Analysis was done on one of the fainter lines to show the full range. At higher attenuations there are some deviations from linear behavior. 

One fiber fit gives the result for fitting one fiber (14 in the overview.png image).  I checked that results are quite similar across different fibers (you can see that also in the figure, where I also show data for 3 other fibers) and for 2 different lines.

One fiber fit_bright_check.png just shows that result which I derived from the faint line (in order to cover the whole range of attenuator values) are consistent with the values determined from the very bright line which can only be analyzed at high levels of attenuation.